Constant-Speed, Constant-Volume AHU
Understand how the constant-speed, constant-volume AHU works. Explore component overviews, measurements, and additional resources.
Overview
A constant-speed, constant-volume (CSCV) air handling unit (AHU) provides conditioning to mixed air (a combination of outdoor and return air), discharges the conditioned air into zones, returns air from the zones, and exhausts air to the outdoors. The CSCV system has a constant volumetric airflow rate. The discharge air is conditioned by heating or cooling heat exchangers that are supplied from the heating and cooling loop systems (e.g., steam loop, hot water heating loop, chilled water loop, refrigerant loop). For a more detailed description of the system, refer to the ASHRAE HVAC Systems and Equipment Handbook (2020), Section 4.
The major components of a CSCV system are fans and motors, heat exchanger coils, and dampers. In general, the mixed air is conditioned by heating or cooling coils that use the output of the heating or cooling loops, with mechanical valves regulating the energy introduced to the coils. Supply and return fans circulate air to and from the zones served, while dampers control airflows.
Components
Fans and Motors
The fan motors provide pressure in the system to move air through the duct work. Typically, fans are placed on the return and supply side of the unit to maintain proper airflow. The air is either blown or pulled through filters and heat exchanger coils, depending on the configuration. In a CSCV system, the motors operate in an on/off manner.
Air Coils
The air coils include a set of heating and cooling coils which provide heating or cooling to the air before it is discharged from the AHU. Mechanical valves regulate the amount of energy introduced from the loop to the air coils, which control the energy supplied to the discharge air.
Dampers
Dampers are generally interlocked and are controlled to supply appropriate quantities of fresh air to the AHU and exhaust air to the outdoors. The dampers can also be set to maintain desired building pressures.
How do I…
Assess System Energy Usage
The primary energy consumption of a CSCV AHU is the electricity that is used to run the fan motors.
Quantify Electricity Usage (kWh) of Single Speed Fans
Measure the hourly average runtime of the fan motors and take spot measurements with a handheld power meter of true RMS power while the motors are running.
How to Measure
Click any measurement below for detailed setup, data collection, and troubleshooting instructions.
Use this technique to measure supply and return fan motor true RMS power with a handheld meter while the motor is running.
Use this technique to measure hourly motor runtime of supply and return fan motors.
Borrow Equipment and Download Calculator
Reserve loggers, download the calculator, and enter your measured data.
CSCV Fan Motor Energy (Runtime) Calculator
Provides estimated annual electricity consumption of a supply and return fan that operate at constant-speed, constant-volume using motor runtime data.
For more details about the methodology behind this calculator, see Fan Motor Energy Consumption.
Download CalculatordownloadMeasurement Locations
Quantify Electricity Usage (kWh) of Two-speed Fans using Motor Current
Measure the average hourly current, take spot measurements of current and true RMS power at low and high speeds with a handheld power meter while the motors are running.
How to Measure
Click any measurement below for detailed setup, data collection, and troubleshooting instructions.
Use this technique to measure true RMS power, current, voltage and power factor of supply and return fan motors at high and low speeds.
Use this technique to measure hourly motor current draw of supply and return fan motors.
Borrow Equipment and Download Calculator
Reserve a kit, download the calculator, and enter your measured data.
CSCV Two-speed Fan Motor Energy (Current) Calculator
Provides estimated annual electricity consumption of supply and return fans that operate at constant-speed, constant-volume and have two operating speeds (high and low) using electrical current data.
For more details about the methodology behind this calculator, see Fan Motor Energy Consumption.
Download CalculatordownloadMeasurement Locations
Quantify Electricity Usage (kWh) of Two-speed Fans using Motor Speed
Measure the hourly fan speed and take spot measurements of true RMS power while the motors are running at low and high speeds.
How to Measure
Click any measurement below for detailed setup, data collection, and troubleshooting instructions.
Use this technique to measure the fan motor true RMS power with a handheld meter.
Motor Speed data from the BAS
If the BAS can trend the motor speed data, download it and resample to hourly intervals.
Borrow Equipment and Download Calculator
Reserve a kit, download the calculator, and enter your measured data.
CSCV Two-speed Fan Motor Energy (Speed) Calculator
Provides estimated annual electricity consumption of constant-speed, constant-volume fans with two operating speeds (high and low) using electrical spot measurements and speed data.
For more details about the methodology behind this calculator, see Fan Motor Energy Consumption.
Download CalculatordownloadMeasurement Locations
Assess System Performance
The performance of a CSCV AHU can be determined by quantifying the heating/cooling load of the zones served by the system.
Quantify Amount of Heat/Cooling Provided to the Space (Btu)
Determine the heating/cooling load on building.
Equipment to Measure
AHU and Ductwork
What to Measure
- Average hourly air flow rate (cfm) of supply air.
- Average hourly air temperature (°F) before the heating/cooling coil.
- Average hourly air temperature (°F) after the heating/cooling coil.
Measurement Locations
Further Reading
ASHRAE (2014). “ASHRAE Guideline 14-2014 – Measurement of Energy, Demand, and Water Savings.” Annex A.
ASHRAE (2020). “ASHRAE Handbook: HVAC Systems and Equipment,” Chapter 1. HVAC SYSTEM ANALYSIS AND SELECTION. I-P Edition.
ASHRAE (2020). “ASHRAE Handbook: HVAC Systems and Equipment,” Chapter 4. AIR HANDLING AND DISTRIBUTION. I-P Edition.